Agreed. Walker has been just fine this year. He's pitched nine appearances in relief before coming back into the rotation and only allowed runs in 3 of them. His two starts since returning - one good, one mediocre. He's got a 3.75 ERA for the year.
He's the fifth starter temporarily and the swing guy when Nola comes back That is well above league average performance for that role.
The money is sunk cost. Just worry about helping the team win (or rather, not lose, from that role. Which he is doing.